1. Field of the Invention
The present invention relates to an automatic steering control apparatus for a vehicle which performs automatic steering control, such as automatic parking control, for automatically steering steerable wheels of the vehicle without requiring the driver to perform steering wheel operation.
2. Description of the Related Art
Conventionally, there have been known various steering systems, such as a steering assisting apparatus for assisting steering operation of steerable wheels of a vehicle in accordance with torque with which a driver operates the steering wheel, and a steering drive apparatus (a so-called steer-by-wire system) for drivingly steering steerable wheels of a vehicle in accordance with an amount by which a driver operates the steering wheel (see, for example, Japanese Patent Application Laid-Open No. 2000-190863).
In a vehicle to which such a steering system is applied, when an anomaly occurs in, for example, a steering actuator for drivingly steering steerable wheels, difficulty is encountered in steering the steerable wheels through only the driver's steering wheel operation. In view of this, the steering system disclosed in the publication is configured such that when an anomaly occurs in, for example, the steering actuator, a difference is produced between braking force acting on a left-hand wheel of the vehicle and that acting on a right-hand wheel of the vehicle on the basis of torque with which the driver operates the steering wheel (hereinafter may be referred to as “steering-wheel operation torque”).
With this operation, a yawing moment corresponding to the direction of the steering-wheel operation torque is generated in the vehicle, and because of this yawing moment, a steering moment corresponding to the direction of the steering-wheel operation torque is generated in the steerable wheels. As a result, the steering operation of the steerable wheels is assisted, and the vehicle can be turned to a direction corresponding to the driver's intended steering operation.
Incidentally, in recent years, there has been proposed an automatic steering control apparatus which performs automatic steering control, such as automatic parking control for automatically steering steerable wheels of the vehicle, without requiring the driver's steering wheel operation, to thereby move the vehicle to a parking position desired by the driver. The force required for drivingly steering the steerable wheels during execution of the automatic steering control changes depending not only on vehicle-related factors such as tire pressure of the steerable wheels, but also on factors related to an environment of the vehicle such as road surface friction coefficient.
Accordingly, depending on the designed specifications of the steering actuator for drivingly steering the steerable wheels, in some cases the steering drive force becomes insufficient even if the steering actuator is normal. In such a case, the actual steering angle of the steerable wheels does not follow the target steering angle, so that the expected steering control cannot be performed. Therefore, such an automatic steering control apparatus is desirably provided with means for compensating for insufficiency of the steering drive force of the steering actuator when the steering drive force is insufficient.
The above-described publication neither discloses nor suggests a technique for compensating for insufficiency of the steering drive force of the steering actuator when the steering drive force of the steering actuator in a normal state becomes insufficient during performance of the automatic steering control.